An electrostatic image may be created by charging a photoconductive layer with a uniform electrostatic charge and thereafter imagewise discharging the electrostatic charge by exposing it to radiant energy in the non-image areas. It will be understood that other methods may be employed to form an electrostatic image, such as, for example, by imagewise transfer of a preformed electrostatic charge to the surface of a carrier that has a dielectric surface. Moreover, the charge may be formed from an array of styluses.
After the latent electrostatic image has been formed, the image may be developed by applying to the latent electrostatic image a liquid developer composition comprising a dispersion of toner particles (which may be pigmented) and, preferably, a charge director. Development takes place when the toner particles electrostatically adhere to the imagewise charge on the photoconductor and are removed from the non-charged areas. In the so-called reversal development process the charged toner particles adhere to the non-charged areas of the photoconductor.
The carrier liquid in which the toner particles, charge director and other ingredients in the developer composition are dispersed is an insulating, nonpolar liquid having and a high-volume resistivity in excess of 10.sup.9 ohm-centimeters, and a low dielectric constant, i.e., below about 3.0. Suitable carrier liquids in which the toners may be dispersed include aliphatic, isomerized hydrocarbons such as the aliphatic hydrocarbons sold by the Exxon Corporation under the trademark ISOPAR. Different types of ISOPARs are available having different distillation end points and vapor pressures. Light mineral oils which are higher boiling aliphatic hydrocarbon liquids may also be used.
After the latent image on the photoconductive layer has been developed the toner particles are transferred imagewise to a carrier sheet where the particles adhere imagewise to the sheet. For example, the developed image may be transferred to a sheet of paper, and the toner image may thereafter be fixed. It has been observed that during the transfer step the toner image may be smudged, smeared, or squashed thereby reducing the resolution, sharpness, line acuity--that is, edge acuity--and clarity of the final image. Furthermore, the toner images made with liquid toner compositions of the prior art are not completely transferred from the photoconductor to the carrier sheet. A result of such incomplete toner transfer is that the image on the carrier sheet evidences image defects, such as, low optical density, pinholes, non-uniformity in the image areas, hollow characters in the image areas, and the like. Moreover, leaving a residue of toner particles on the photoconductor presents problems for cleaning the engine of the electrostatic reproduction machine prior to the next cycle of image reproduction on the photoconductor. Residual toner on the photoconductor during the next cycle will result in a dirty image on the carrier sheet.
With specific references to said Ser. No. 679,906, the toner particles are prepared from certain thermoplastic polymer resins by processes that provide the toner particles with a morphology of a plurality of fibers as defined therein. The fiber toner particles may interdigitate, intertwine, or interlink physically providing a developed, transferred, squash-resistant image having superior sharpness, line acuity and a high degree of resolution. The salient feature of the toner particles and the developed images made with such toners is that the particles and images have good compressive strength thereby allowing the developed images to be transferred from the photoconductor surface on which they are developed to carrier sheets without being squashed.
The thickness of the developed image mass can be controlled by varying any one or more of the charge on the photoconductor, the development time, the concentration of toner particles in the developer dispersion, the charge characteristics of the toner particles, the toner particle size, and the surface chemistry of the toner particles. Because of the intertwining property of the toner particles, as described in said Ser. No. 679,906, a thicker developed image may be created on the photoconductor layer which is then transferred to the carrier sheet, and yet the toner image on the carrier sheet is very sharp. Accordingly, the fiber toner particles of Ser. No. 679,906 provide thicker developed images without the need to resort to the procedures of the prior art that may result in other disadvantages such as more complex machines, additional and expensive chemicals, longer times to make copies, poorer image quality, and the like.
The fiber toner particles of Ser. No. 679,906 may be prepared by dispersing or dissolving a pigment in a plasticized polymer at a temperature of between about 65.degree. C. and 100.degree. C., allowing the dispersion to solidify and form a sponge, and then grinding the sponge to the appropriate particle size and fiber morphology. It is estimated that the average particle size is in the range of about 1-3 microns. Another method is to dissolve one or more thermoplastic polymers in a nonpolar dispersant liquid together with a pigment, such as carbon black or the like, and thereafter the mixture is allowed to cool slowly with stirring during which time the pigmented fiber toner particles precipitate. In a preferred embodiment, the precipitated particles are ground to provide the desired particle size as well as allowing further dispersion of the pigment in the polymer. A third method is to heat a thermoplastic polymer above its melting point and disperse therein a pigment, and thereafter the pigmented thermoplastic polymer is pulled apart by grinding to form fiber particles without first forming a sponge. Dyes may be used in addition to or in place of pigments.
The fibrous toner particles, formed by any of the foregoing methods, are dispersed in a nonpolar carrier liquid, together with one or more charge directors to form the liquid toner/developer composition. Charge directors are well known in the art. Charge directors, which are well known in the field of liquid toner/developer compositions, must be soluble or dispersible in the carrier liquid dispersant and should cause charging of the imaging toner particles. Examples of charge directors are basic barium petronate and other petronates, di-tridecyl sodium sulfosuccinate (known as Aerosol TR) and other sulfosuccinates, soybean lecithin, cobalt octoate and other octoates, cobalt naphthanate and other naphthanates, and the like.
The thermoplastic polymers of Ser. No. 679,906 include ethylene vinyl acetate copolymers sold by DuPont under the ELVAX trademark and ELVAX II resins which are ethylene copolymers combining carboxylic acid functionality, high molecular weight, and thermal stability. Other polymers which are usable are isotactic polypropylene (crystalline), polybutyl terephthalate, the ethylene ethyl acrylate series sold by Union Carbide under the trademark BAKELITE, other ethylene vinyl acetate resins, methacrylate resins, such as polybutyl methacrylate, polyethyl methacrylate, and polymethyl methacrylate, polyvinyl chloride and polyamides. Plasticizers may be included in the polymeric compositions.
The polymers may be pigmented or dyed so as to render the developed image visible. The pigment may be present in the amount of about 3 percent to about 60 percent by weight of the polymer. If a dye is used, it may be present in an amount of between 1 percent or lower and about 25 percent by weight of the polymer. If no colorant is used--as, for example, in making a toner for developing a latent image for a printing plate--a minor amount of silica, such as Cabosil, may be added to make the grinding easier.